Add "devMenu=1" to the "[Game]" section of Settings.ini, then press ESC in game.

Nice find! Is there anything interesting in the options beyond what's available in the regular game?

You can access the remnants of the Coming Soon screen You can also play Media demo mode which is just Green Hill Zone followed by a buggy, muted Studiopolis Act 1. I haven't been able to get through that version of Studiopolis without the game crashing though. Also another mod:

It's literally a sprite hack that puts Knuckles sprites over Sonic's, resulting in a Knuckles that has all of Sonic's abilities and can have Tails follow him... so that's kinda what I wanted

It shows up in No Save mode. You get one by getting a Mach Bonus (that is, collect enough Blue Spheres when you're already in Mach 3). And when you use a Continue, you start from the Act you died on instead of the Zone.